Transition Metal Halogenides as Visi - ACS Publications - American

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Novel Three-Dimensional Semiconducting Materials Based on Hybrid d10 Transition Metal Halogenides as Visible Light-Driven Photocatalysts Cheng-Yang Yue,†,‡,§ Bing Hu,*,‡ Xiao-Wu Lei,*,†,‡ Rui-Qing Li,† Fu-Qi Mi,† Hui Gao,† Yan Li,† Fan Wu,† Chun-Lei Wang,† and Na Lin† †

Key Laboratory of Inorganic Chemistry in Universities of Shandong, Department of Chemistry and Chemical Engineering, Jining University, Qufu, Shandong 273155, P. R. China ‡ State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, P. R. China § Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), Xiamen University, Xiamen, Fujian 361005, P. R. China S Supporting Information *

ABSTRACT: The development of new visible light-driven photocatalysts based on semiconducting materials remains a greatly interesting and challenging task for the purpose of solving the energy crisis and environmental issues. By using photosensitive [(Me)2-2,2′-bipy]2+ (1,1′-dimethyl-2,2′-bipyridinium) cation as template, we synthesized one new type of inorganic−organic hybrid cuprous and silver halogenides of [(Me)2-2,2′-bipy]M8X10 (M = Cu, Ag, X = Br, I). The compounds feature a three-dimensional anionic [M8X10]2− network composed of a one-dimensional [M8X12] chain based on MX4 tetrahedral units. The photosensitization of organic cationic templates results in narrow band gaps of hybrid compounds (1.66−2.06 eV), which feature stable visible lightdriven photodegradation activities for organic pollutants. A detailed study of the photocatalytic mechanism, including the photoelectric response, photoluminescence spectra, and theoretical calculations, shows that the organic cationic template effectively inhibits the recombination of photoinduced electron−hole pairs leading to excellent photocatalytic activities and photochemical stabilities.

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INTRODUCTION It is well-known that silver halides are extensively used as photosensitive materials in photographic films because of their excellent photosensitivity.1 Of course, because of their photochemical instability in sunlight, which is true for the principle of chemical photography, silver halide cannot be independently used as a photocatalyst. However, it is reported that Ag0 atoms formed in the previous photoreduction period will engulf the AgX and prevent further photoreduction of AgX.2 Subsequently, the Ag/AgX plasmonic photocatalysts were widely studied and exhibited excellent photocatalytic activities and photochemical stabilities because of the surface plasma resonance effect of the Ag0 nanoparticle and the Schottky barrier formed between the Ag and AgX components.3−5 It should be noted that the strong separating ability of the Ag nanoparticle for the photoinduced carriers plays an important role in the photocatalytic activities and photochemical stabilities of the Ag/AgX photocatalyst.3a That is, the introduction of electron-separating components into silver halogenides can effectively accelerate the separation process of © 2017 American Chemical Society

photoinduced carries and decrease the electron densities of AgX components, which will prevent the photolysis reactions of AgX and increase their photochemical stabilities. However, as part of these strategies, understanding the structure−property relationship is a significant challenge because of the mixed compositions without clear crystal structures. For this reason, building a new structural mode based on metal halogenide as a stable visible light-driven photocatalyst is a greatly interesting and challenging task for investigating the electronic band structure and possible electronic interactions based on a singlecrystal structure. In recent years, a large number of organic cation-directed hybrid metal halogenide materials have received a great deal of attention because of their distinctive photoelectric properties.6−10 It is found that the template or structure-directing agent (SDA) occupies a very significant position in the crystal structure construction and band structure decorating as well as Received: May 10, 2017 Published: August 28, 2017 10962

DOI: 10.1021/acs.inorgchem.7b01171 Inorg. Chem. 2017, 56, 10962−10970

Article

Inorganic Chemistry

CuI. The orange crystals of 2 were obtained with a yield of 18% based on AgI. Elemental analytical calculated for C12N2H14Ag8I10 (2): C, 6.22; H, 0.61; N, 1.21. Found: C, 6.29; H, 0.70; N, 1.12. Synthesis of Compound [(Me)2-2,2′-bipy]Cu8Br10 (3). A mixture of KBr (1 mmol), CuBr (10 mmol), 2,2′-bipy (1 mmol), hydrobromic acid (2 mL), methanol (4 mL), and deionized water (2 mL) was sealed in a 15 mL poly(tetrafluoroethylene)-lined stainless steel reactive container. The mixture was first reacted at 140 °C for 6 days and then slowly cooled to room temperature at a cooling rate of 5 °C min−1. After the filtration, block-shaped orange crystals were obtained and subsequently determined to be [(Me)2-2,2′-bipy]Cu8Br10 (3). The crystals were easily separated by hand under a microscope with a yield of 19% based on CuBr, and they were finally washed with ethanol and distilled water. Elemental analytical calculated for C12N2H14Cu8Br10 (3): C, 9.65; H, 0.94; N, 1.88. Found: C, 9.72; H, 0.99; N, 1.77. Crystal Structure Determinations. Single-crystal data of all the title compounds were collected on a Bruker Apex II CCD diffractometer equipped with Mo Kα radiation (λ = 0.71073 Å) at 293(2) K. The crystal structures were determined and refined on F2 by using SHELXS-97 and SHELXL-97.15 Anisotropic thermal parameters were refined for all the non-hydrogen atoms, and the H atoms of [(Me)2-2,2′-bipy]2+ cations were positioned theoretically on the specific carbon atoms and refined isotropically using a rigid model. It should be noted that the Ag(4) atom slightly split into two sites with partial occupancies of 0.89(3) and 0.11(3) for Ag(4a) and Ag(4b) positions, respectively, in compound 2. It is reported that silver atoms always feature disorder or relatively large displacement parameters, and similar phenomena have also been observed in many hybrid silver halogenides.13a The single-crystal data of four compounds are summarized in Table 1, and bond distances are listed in Tables S1− S3. CCDC 1497143−1497145 contain supporting crystal data for 1− 3, respectively. Infrared (IR) and Solid Nuclear Magnetic Resonance (NMR) Spectrum. To confirm the organic cations of [(Me)2-2,2′-bipy]2+ in hybrid compounds, the solid NMR and IR spectra of compound 1 were examined. 1H CP NMR (400 MHz) and 13C CP NMR (100 MHz) spectra were recorded using a 400 MHz spectrometer. Chemical shifts are reported in parts per million. Multiplicities are indicated as s (singlet), d (doublet), t (triplet), q (quartet), and m (multiplet): 1H CP NMR (400 MHz) δ 9.98 (s, 1H), 5.86 (s, 2H), 5.09 (s, 5H), 4.07 (s, 2H), 2.01 (s, 6H); 13C CP NMR (100 MHz) δ 151.78, 149.77, 141.89, 133.37, 52.63 (Figure S2); IR (KBr) 3440 (m, C−H), 1620 (w, CN) and 1430 (s, CC), 1280(s, C−H), 768 (w, C−H) cm−1 (Figure S3). Photocatalytic Activity Measurement. The purities of powder samples were examined by PXRD and XPS analyses. Subsequently, the photocatalytic properties of powder samples were studied via the decomposition of rhodamine B (RhB) and methyl orange (MO) as model organic pollutants. In a typical process, each powder sample of the compound (30 mg) was added to the 30 mL RhB or MO solution (1 × 10−5 mol L−1). The mixture was dispersed in an ultrasonic bath for ∼30 min and then constantly stirred in the dark overnight before photocatalytic reaction to ensure the adsorption balance between the powder sample and organic pollutant molecule. Subsequently, the solution was irradiated under visible light from a Xe lamp (50 W) with a distance of 10 cm between the Xe lamp and the surface of the reaction solution. In the Xe lamp, the cutoff filter was used to inhibit the UV (780 nm) light, ensuring irradiation with only visible light. In the photocatalytic reaction, ∼4 mL of the mixture was continually extracted from the reaction container at a given interval time and the sedimentary powder samples were separated from the suspensions by using high-speed centrifugation. The degradation process of RhB and MO was monitored by examining the intensity variation of maximal absorption in the UV−vis spectra at 552 and 464 nm, respectively. Photocatalytic stability was examined using the re-collected samples from the former experiments by centrifugation, separation, and drying at 80 °C for 8 h. The radical trapping experiments were performed by adding a small amount of

photoelectric property regulation of hybrid metal halogenide materials.11 These surprising discoveries incite us to explore new types of stable visible light-driven photocatalysts via the introduction of conjugated organic cations as photosensitive templates and electron acceptors into binary CuX or AgX based on the idea of electronic band structural decoration. On one hand, the organic cation can strongly tune the electronic band structures of hybrid metal halogenides within broad sunlight absorption ranges, which has been proven by many reported works.12 On the other, the photosensitive organic cations feature excellent electron accepting and transfer capabilities for absorbing the photoinduced electrons, which effectively prevents the reduction of Ag+ or Cu+ ions.13 Hence, the photosensitive organic cations directed hybrid metal halogenides afford another innovative design strategy to prepare new type of visible light driven photocatalysts with advantages of higher photocatalytic activities and photochemical stabilities than binary metal halogenides. Such a system also helps us to investigate the electronic band structure and possible electron transfer route based on a definite crystal structure, which is useful for understanding the possible photocatalytic mechanism. Herein, using in situ-synthesized [(Me)2-2,2′-bipy]2+ as a photosensitive cationic template, we obtained one new type of cuprous and silver halogenide, namely, [(Me)2-2,2′-bipy]M8X10 (M = Cu or Ag; X = Br or I) with three-dimensional (3D) anionic [M8X10]2− networks through a solvothermal reaction. To the best of our knowledge, these materials represent the first type of 3D hybrid d10 metal halogenide with visible light-driven photocatalytic properties, which also affords the first structural mode for the study of host−guest electronic interplay and the photoinduced carrier migration mechanism.

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EXPERIMENTAL SECTION

Materials and Instruments. All the reagents and solvents were commercially purchased and directly used without further chemical purification. The N-doped TiO2 (P25) was prepared using the previously reported method in which Degussa P25 was heated at 550 °C for 3 h in a NH3/Ar (2:1) atmosphere.14 The N concentration was 0.99 atom % based on X-ray photoelectron spectroscopy (XPS). Powder X-ray diffraction (PXRD) data were collected with a Bruker D8 ADVANCE powder X-ray diffractometer (Cu Kα, λ = 1.5418 Å) with a 2θ range of 5−80°. Ultraviolet−visible (UV−vis) absorption was measured with a PE Lambda 900 UV/vis spectrophotometer (200−800 nm). Thermogravimetric analysis (TGA) was performed on a Mettler TGA/SDTA 851 thermal analyzer at a heating rate of 10 °C min−1 under a nitrogen atmosphere. The metal compositions and oxidation states of all the compounds were studied by XPS using monochromatized Al Kα X-ray radiation (ThermoFisher Scientific Co., ESCALAB 250). The solid-state photoluminescence spectra were recorded on a FLS920 fluorescence spectrophotometer. Syntheses of Compounds [(Me)2-2,2′-bipy]Cu8I10 (1) and [(Me)2-2,2′-bipy]Ag8I10 (2). A mixture of KI (1 mmol), CuI (10 mmol), 2,2′-bipy (1 mmol), hydriodic acid (2 mL), methanol (4 mL), and acetonitrile (2 mL) was sealed in a 15 mL poly(tetrafluoroethylene)-lined stainless steel reactive container, which was constantly reacted at 140 °C for 6 days. After the reaction, the system was cooled to room temperature at a cooling rate of 5 °C min−1 and the mixture was then filtered. A large amount of blockshaped dark red crystals were found and subsequently determined to be [(Me)2-2,2′-bipy]Cu8I10 (1) by X-ray single-crystal diffraction. The crystals were easily collected by hand with a yield of 50% based on CuI material and washed with ethanol and distilled water. Elemental analytical calculated for C12N2H14Cu8I10 (1): C, 7.34; H, 0.72; N, 1.43. Found: C, 7.39; H, 0.80; N, 1.35. Isotypical compound 2 was also prepared via a method similar to that used for 1 with AgI instead of 10963

DOI: 10.1021/acs.inorgchem.7b01171 Inorg. Chem. 2017, 56, 10962−10970

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Inorganic Chemistry

10−5 eV, and the default values of the CASTEP code were adopted for the other calculation parameters.

Table 1. Single-Crystal Data and Structural Refinement Details for Compounds 1−3 chemical formula fw space group a (Å) b (Å) c (Å) β (deg) V (Å3) Z Dcalcd (g cm−3) temp (K) μ (mm−1) F(000) no. of reflections collected no. of unique reflections no. of reflections [I > 2σ(I)] goodness of fit on F2 R1, wR2 [I > 2σ(I)]a R1, wR2 (all data) Δρmax (e/Å3) Δρmin (e/Å3)

1

2

3

C12N2H14Cu8I10

C12N2H14Ag8I10

C12N2H14Cu8Br10

1963.57 P21/c (No. 14) 18.2341(12) 11.4990(8) 15.6652(10) 95.7730(10) 3267.9(4) 4 3.991 293(2) 14.557 3448 37261

2318.21 C2/c (No. 15) 18.8988(17) 11.8180(10) 15.9042(14) 95.8110(10) 3533.9(5) 4 4.357 293(2) 13.081 4024 20005

1493.67 C2/c (No. 15) 17.617(2) 11.1717(13) 15.0247(17) 97.3510(10) 2932.7(6) 4 3.383 293(2) 19.317 2728 16673

7495

4055

3332

6401

3273

2719

1.028

1.020

1.010

0.0252, 0.0525

0.0508, 0.1448

0.0647, 0.1911

0.0329, 0.0550

0.0630, 0.1541

0.0777, 0.2012

2.970 −2.144

2.140 −3.567

3.606 −2.152

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RESULTS AND DISCUSSION Compounds 1−3 were solvothermally prepared with CuI (AgI or CuBr), 2,2-bipy, and KI (KBr) as the starting materials in a mixed solution of HI (HBr) and methanol. In the reaction, the [(Me)2-2,2′-bipy]2+ cations were obtained from the nucleophilic substitution reaction of organic amine (2,2′-bipy) and methyl iodide (CH3I) or methyl bromide (CH3Br), whereas CH3I or CH3Br was obtained from methanol (CH3OH) activated by HI or HBr under acidic conditions. A similar phenomenon has also been found for a large number of inorganic−organic hybrid materials.17 Crystal Structures. Compound [(Me)2-2,2′-bipy]Cu8I10 (1) crystallizes in monoclinic space group P21/c (No. 14), whereas both [(Me)2-2,2′-bipy]Ag8I10 (2) and [(Me)2-2,2′bipy]Cu8Br10 (3) belong to space group C2/c (No. 15). Compound 1 features a 3D [Cu8I10]2− anionic network with one-dimensional (1D) large tunnels occupied by [(Me)2-2,2′bipy]2+ cations (Figure 1b). The asymmetrical unit contains eight crystallographically independent Cu+ ions, 10 I− ions, and one [(Me)2-2,2′-bipy]2+ cation. All the Cu+ ions are surrounded by four I− ions in slightly distorted tetrahedral coordination environments with normal Cu−I bond distances of 2.6132(8)− 2.8011(11) Å.10a,b,13b All the CuI4 tetrahedra are interconnected via edge or corner sharing to form a 1D [Cu8I12] chain down the c-axis, which are further interlinked with four others via sharing four terminal iodine atoms in a novel 3D [Cu8I10]2− anionic network (Figure 1a). As a result, the 3D [Cu8I10]2− network contains a large [Cu12I12] 24-member 1D tunnel with a quadrangular cross section of 6.352 × 12.043 Å2 presented along the c-axis, and the isolated [(Me)2-2,2′-bipy]2+ cations occupy such a 1D tunnel (Figure 1b). At the same time, there are also two types of 1D tunnels along the [110] and [101] directions, which are also occupied by [(Me)2-2,2′-bipy]2+ cations (Figure S4). The Cu···Cu distances of 2.7044(15)−2.9552(11) Å are comparable to the sum of van der Waals radii (2.80 Å), indicating the existence of weak metal···metal interactions in compound 1.18 Compound 2 features structural characteristics similar to those of compound 1, and there are four crystallographically independent Ag+ ions, five I− ions, and one-half of a [(Me)2-2,2′-bipy]2+ cation. Every two Ag(1)I4,

R1 = ∑||Fo| − |Fc||/∑|Fo|, and wR2 = {∑w[(Fo)2 − (Fc)2]2/ ∑w[(Fo)2]2}1/2.

a

tert-butyl alcohol (TBA), ammonium oxalate (AO), or benzoquinone (BQ) before irradiation for each degradation reaction. Calculation Details. The densities of states (DOS) of compounds 1 and 2 were calculated by using the CASTEP code based on density functional theory (DFT), in which the valence and core electrons were set by the plane-wave basis and norm-conserving pseudopotential, respectively.16 A cutoff energy of 320 eV was used to determine the number of plane waves, and the numerical integration of the Brillouin zone was performed with 2 × 2 × 2 Monkhorst−Pack k-points. The convergence criteria used an energy convergence tolerance of 1.0 ×

Figure 1. Detailed view of (a) the 1D [Cu8I12] chain down the c-axis and (b) the 3D crystal structure of compound 1 down the c-axis. 10964

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Inorganic Chemistry Ag(2)I4, Ag(3)I4, and Ag(4)I4 tetrahedral units are selfcondensed to form a similar 1D [Ag8I12] chain with half of the repeating units along the c-axis and finally form a 3D [Ag8I10]2− anionic network (Figure S5b). It is worth noting that the tetrahedral [CuX4] or [AgX4] units often feature various self-condensation characteristics that allow them to form polyanionic units of a cluster or oligomer as secondary building units (SBUs). However, most of the SBUs are apt to interconnect to form a 1D anionic chain and occasionally a two-dimensional (2D) layer. In relative terms, 3D anionic networks templated by organic cations have rarely been reported in hybrid metal halogenides until now, including limited phases of [Cu6I8]2−, [AgI2]−, [Ag2I4]2−, [Ag4I6]2−, [Ag5I6]−, and [Ag13I17]4−.8,9 In addition, similar [Cu8X10]2− units are also reported in some hybrid metal halogenides but with completely different structural types, such as [Cs2(18c6)3][Cu8I10(MeCN)2] and Cu8X10(H-quinine)2 (X = Cl or Br). 19 In the former compound, the finite [Cu8I10(MeCN)2]2− unit is composed of four [CuI3] triangles, two [CuI4] tetrahedra, and two [CuI3(MeCN)] tetrahedra, while in the latter phase, the [Cu8X10] unit is built from the linear [CuX2], trigonal [CuX3], pyramidal [CuX5], and two terminal Cu atoms, which are further coordinated and linked by H-quinine ligands to form a 2D Cu8X10(H-quinine)2 layer. Undoubtedly, the [Cu8I10]2−, [Ag8I10]2−, and [Cu8Br10]2− reported here represent one new type of 3D network with the largest regular tunnel in hybrid metal halogenide chemistry. The PLATON analysis suggests a solvent-accessible volume of approximately 37% of the total crystal volume based on only the inorganic [Cu8I10]2− framework. The BET surface area of compound 1 was 4.4 cm2/g based on N2 adsorption analysis (Figure S6). TGA shows that the title compounds are stable until ∼200 °C (Figure S7). Crystalline title compounds are also stable in air, DMF, acetonitrile, ethanol, methanol, and other polar organic solvents. Optical Properties. At room temperature, we measured the solid-state absorption optical spectra of powder samples 1−3 (Figure 2). The UV−vis absorption optical spectrum shows that all the compounds feature strong absorptions from ultraviolet to visible light regions. Via extrapolation of the linear portion of the absorption edge, the intercept of the tangent line gives band gaps of 1.66 eV (1), 1.92 eV (2), and 2.06 eV (3). Obviously, it is very easy to regulate the band gaps of such semiconducting materials by adopting different

inorganic frameworks. Furthermore, all the title compounds (1−3) show an obvious red shift of the absorption edge and feature smaller band gaps compared with the primary band gaps of CuI (2.95 eV), β-AgI (2.81 eV), and CuBr (2.89 eV), respectively. Reportedly, the conduction bands of most conjugated organic cation-directed hybrid metal halogenide compounds are occupied by organic cations, and the valence bonds are contributed by a metal halogenide anionic network based on the electronic band structural calculation.7a It is the contribution of conjugated organic cations to conduction bands that leads to the red shift of the absorption edge as determined by comparison of the binary metal halogenide (CuX and AgX). Hence, the optical absorption of title compounds 1−3 may be ascribed to the intermolecular charge transfer (CT) between metal halogenide anions and organic cations, and a similar phenomenon is also reported for a series of hybrid metal halogenides, such as (H2dpe)0.5(β-AgI2), (Hpy)(Ag5I6), [N-BzPy] 2 [Cu 6 I 8 ], [MC][Ag 2 I 3 ], [EC][Ag 2 I 3 ], [PC] 2 [Ag 4 I 6 ], [BC]2[Ag4I6], etc.7a,9b,12,20 Photocatalytic Properties. The narrow band gap characterizations of these hybrid materials incite us to study their photocatalytic properties, which were evaluated by photodegradations of RhB and MO as the testing organic pollutants at room temperature. Before the photocatalytic experiments, not obvious decrease in RhB and MO concentration was found after reaction for 10 h during the photolysis experiment without our samples or the catalytic experiment without visible light irradiation. On the contrary, under the photodegradation action of 1 and 2, the purplish red color of the RhB solution gradually changes to colorless and the concentration of organic molecules continuously decreases under visible light irradiation (Figure S8). After visible light irradiation for 80 min, the degradation ratios of RhB reach 98 and 95% for 1 and 2, respectively, indicating almost complete decomposition of organic pollutants (Figure 3a). For the MO dye, 1 can nearly completely decompose MO within 30 min, whereas the same MO solution is almost completely decomposed by 2 within 60 min (Figure S9). For comparison, we also investigated the photocatalytic activity of N-doped TiO2 as a reference under the same reaction conditions, and the results indicate that ≳44 and 70% of RhB and MO molecules are still not decomposed after visible light irradiation for 80 and 30 min, respectively (Figure 3a,c). Furthermore, the CuI shows no observable photodegradation for MO and RhB, and AgI shows slight photodegradation effects that may be due to the formation of a Ag@AgI plasmonic photocatalyst during the reaction. Obviously, both 1 and 2 have photodegradation activities higher than those of N-doped TiO2 and primary CuI or AgI. Considering the fact that most of the dye degradations belong to pseudo-first-order kinetic reaction, we calculate the rate constants of RhB dye degradation over two compounds to be 0.0389 min−1 for 1 and 0.0284 min−1 for 2, which are obviously higher than that of N-doped TiO2 (0.0070 min−1) (Figure 3b). Similarly, the rate constants for MO dye degradation over compounds 1 and 2 (0.112 and 0.0297 min−1, respectively) are of 10 and 2.5 times than that of Ndoped TiO2 (0.0118 min−1), respectively (Figure 3d). Furthermore, the photocatalytic activity of 1 is greater than that of 2, whereas their band gaps are in the reverse order. Such a phenomenon has also been reported for many photocatalysts, which may be due to the stronger absorption of visible light for narrow band semiconductors. It should be noted that 3 features strong absorption for MO and RhB solution, and we have no

Figure 2. Solid-state absorption optical spectra of compounds 1−3. 10965

DOI: 10.1021/acs.inorgchem.7b01171 Inorg. Chem. 2017, 56, 10962−10970

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Figure 3. Visible light-driven photocatalytic degradations of (a) RhB and (c) MO over compounds 1 and 2 and the linear relationship between ln(C/C0) and reaction time for (b) RhB and (d) MO.

cuprous and sliver ions. To gain insight into this puzzle and simultaneously research the possible photocatalytic mechanism, we first conducted radical trapping experiments by using different scavengers over compound 1; for example, tert-butyl alcohol (TBA) captures •OH radical, ammonium oxalate (AO) catches the hole (h+), and benzoquinone (BQ) traps the •O2− radical. The results show that introduction of TBA has little effect on the photocatalytic activity of compound 1; however, the addition of AO and BQ obviously prevents the photodegradation reactions (Figure S13). This illustrates that the h+ and •O2− radicals are the dominant reactive species that play indispensable roles in photodegradation of organic dyes for 1. A similar phenomenon is also found for compound 2. Furthermore, we also studied the electronic band structures of compounds 1 and 2 by performing DFT calculations. The calculated total and partial DOS from each element are shown in Figure 5. For compound 1, the valence bands (VBs) in the energy range of −3 to 0 eV are essentially dominated by the I 5p and Cu 3d electrons, and the conduction bands (CBs) in the range of approximately 2−3 eV are mainly contributed by the C 2p and N 2p electrons. A similar band structure is also observed for compound 2. Hence, the charge transition of compounds 1 and 2 mainly occurs between the 3D anionic frameworks and organic cations.

way to compare its photocatalytic activity in spite of obvious photodegradation activity. Except for the photocatalytic activities, the recyclability of the photocatalyst also can be attributed to a basic requirement for the potential application. Hence, the cyclic photodegradation experiments with the MO dye were further performed over 1 and 2. As shown in Figure S10, the MO dyes quickly decompose in each photodegradation experiment, and the photodegradation rates and efficiencies decrease slightly perhaps because of the loss of the sample during the collection process. The XPS peaks at the end of repeated experiments do not show any obvious change compared with those of the asprepared sample (Figure 4). Furthermore, the XRD patterns are almost identical to simulated data despite some broad peaks or large bumps, which are mainly due to the excessively small amount and inferior crystal quality of the powder sample after photodegradation reactions (Figure S11). Thus, 1 and 2 can be used as stable visible light-driven photocatalysts. Possible Photocatalytic Mechanism. It is well-known that it is impossible that the primary cuprous and silver halogenides can be directly explored as photocatalysts because of their photochemical instabilities under sunlight.2 Hence, as the stable photocatalysts, the photocatalytic reactions of the title materials require an effective method of separating the photoinduced electron−hole pairs to prevent the reduction of 10966

DOI: 10.1021/acs.inorgchem.7b01171 Inorg. Chem. 2017, 56, 10962−10970

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Inorganic Chemistry

Figure 4. XPS spectra of compound 1 [(a) Cu 2p and (b) I 3d] and compound 2 [(c) Ag 3d and (d) I 3d].

Figure 5. Total and partial DOS of (a) 1 and (b) 2. 10967

DOI: 10.1021/acs.inorgchem.7b01171 Inorg. Chem. 2017, 56, 10962−10970

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Figure 6. (a) Transient photocurrent density vs time for compound 1. (b) Photoluminescence spectra of compound 1 as well as primary CuI.

To further investigate the photoinduced carrier characteristics of the hybrid material, we first compared the visible lightdriven photoelectric response of compound 1 and primary CuI. As shown in Figure 6a, the photocurrent trace of the compound 1 electrode shows a rapid response and good reproducibility from the beginning to end of visible light irradiation. Furthermore, the photocurrent intensity of the compound 1 electrode is also obviously higher than that of the CuI electrode. In addition, the photocatalytic activity is greatly influenced by the recombination of photoinduced electrons and holes; that is, a higher recombination ratio will decrease the photocatalytic yield. At the same time, the recombination of photoinduced free carriers always leads to photoluminescence (PL) emission. Hence, the PL spectra can be used as an effective method to evaluate the separation capacity of the photoinduced carriers, and a higher PL intensity corresponds to the less efficient photocatalytic activity. Figure 6b shows the PL spectra of primary CuI and compound 1 generated by UV light excitation (λ = 370 nm). We found that primary CuI gives a strong emission peak at ∼735 nm and weak emission around 422 nm, while compound 1 exhibits very weak emission at 820 nm. Hence, both the higher photocurrent density and weaker PL emission intensity indicate better generation and separation of photoinduced carriers of compound 1 than those of primary CuI. On the basis of the experimental and theoretical results described above as well as the reported similar work, the possible photocatalytic mechanism about the band structure and charge transfer is proposed in Figure 7.21,22 During the photocatalytic reaction, hybrid material 1 is excited by visible light irradiation to produce the photoinduced electron−hole pairs. The holes move to the surfaces of the anionic [Cu8I10]2− network, and the electrons are transferred to the [(Me)2-2,2′bipy]2+ organic cations, which prevents the recombination of electrons and holes. The electrons are further captured by O2 in the solution to form superoxide ion (•O2−), which participates in the degradation of organic dyes. At the same time, the remaining photoinduced holes also contribute to the decomposition of organic dyes. According to our analyses, there are three possible reasons for the enhancement of the photocatalytic stabilities and activities of hybrid halogenides compared with those of primary CuI and AgI. First, the visible light absorption abilities of the

Figure 7. Photodegradation mechanism of organic dyes over the title hybrid materials under visible light irradiation.

hybrid halogenides are better than those of CuI and AgI because of the great contributions of conjugated organic cations to the conduction bands, which have been proven by UV−vis absorption spectrum and band structure calculation. Second, the photoinduced electrons are easily transferred to the conjugated organic cations rather than being localized on the Cu+ or Ag+ ions of anionic networks, which will effectively inhibit the reduction of Cu+ or Ag+ ions. At the same time, the organic cations are also able to effectively prevent the recombination of photoinduced electron−hole pairs based on the photocurrent and PL spectrum. Hence, the introduction of organic cations plays an important role in the construction of the 3D network, band structural regulation, photochemical stabilities, and separation of photoinduced carriers in the photocatalytic degradation process for hybrid halogenides.

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CONCLUSIONS In conclusion, one new type of organic cation-directed cuprous and silver halogenide with a 3D network was prepared and features stable photocatalytic activities under visible light irradiation. The dramatic and stable visible light-driven photocatalytic performances should be attributed to the combination effect of photosensitive organic cations and the 3D network. This work further illustrates the possibility of constructing new visible light-driven photocatalysts based on 10968

DOI: 10.1021/acs.inorgchem.7b01171 Inorg. Chem. 2017, 56, 10962−10970

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Inorganic Chemistry

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hybrid semiconducting metal halogenides. Further studies by our group of the structural regulation and structure−photocatalytic property relationships are in progress.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01171. Crystallographic data in CIF format (CCDC numbers 1497143 for 1, 1497144 for 2, and 1497145 for 3), tables of selected bond distances, thermogravimetric curves, solid NMR and IR spectra, absorption spectra of photodegradative RhB and MO, XRD powder patterns, and XPS data (PDF) Accession Codes

CCDC 1497143−1497145 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Xiao-Wu Lei: 0000-0003-4603-9093 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the financial supports from the National Nature Science Foundation of China (Nos. 21571081, 21671080, and 21601181), Fund of state key laboratory of structural chemistry (Nos. 20150005 and 20170017), NSF of Fujian Province (2016J05054), and the cultivating project for talent team and ascendant subject of university in Shandong Province.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.7b01171 Inorg. Chem. 2017, 56, 10962−10970

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DOI: 10.1021/acs.inorgchem.7b01171 Inorg. Chem. 2017, 56, 10962−10970